Judging by Dr. Grindlay's own remarks, some of the 'flagged' images may have nothing seriously wrong with them, or at least nothing that would invalidate Dr. Schaefer's original findings. It would be well if we could hear from Dr. Schaefer, about the rationale he used in deciding to use, or not use certain 'flagged' images. It's already been established that he took some care to exclude certain problem images, which he believed would affect his results.

I never really agreed that comets can block as much as 22% of a F-type star, so for the past few months I have been studying the possibility of KIC 8462852 being a long period binary, which as far as I can tell wasn't been ruled out by the original kic paper.Here are some intriguing images.http://imgur.com/a/6335i

The dips shown by KIC 8462852 (Tabby’s Star) are still a profound mystery. Further, I have found that Tabby’s star has faded by ~20% from 1890 to 1989 as measured from the Harvard plates. (This is now Schaefer 2016, ApJLett, 822, L34.) A straight line fit to the light curve gives a slope of +0.164 ± 0.013 magnitudes-per-century. The quoted error bar here is from the measurement error (as taken by a chi-square fit), whereas there is some larger systematic error associated with all the usual small problems in photographic photometry and the sampling of the plates.

To measure the systematic errors, I used 12 uncrowded check stars of the same magnitude and color as Tabby’s star, and all within ~22 arc-minutes. The average of the linear slopes is -0.007 mag/century, with an RMS scatter of 0.044 mag/cen. With the systematic error dominating, the century-long decline of Tabby’s Star is significant at the 4.0-sigma level (i.e., a probability of 0.000064 of such a high slope occurring by chance, even with systematic errors).

Two papers (Hippke et al. arXiv:1601.07314v4 & Lund et al. arXiv:1605.02760v1) have recently appeared with the basic claim that the historic light curves from Harvard (as part of the DASCH [Digital Access to a Sky Century@Harvard] database) have a much larger systematic error, more like ±0.15 mag/cen, with the agreed slope for Tabby’s Star then not being anything special. If the DASCH RMS scatter in the fitted linear slopes is really this large, then the existence of the century-long fading in Tabby’s Star would not be significant.

The two papers of Hippke and Lund have been widely publicized, because both authors have run to the press first. Indeed, Hippke contacted at least one reporter *before* he had submitted the first version of his paper. (At that time, Hippke had known about the existence of the Harvard plates for only two weeks, he had talked with zero people who had ever seen any archival plate, and Hippke still has never laid eyes on any archival plate.) In the usual way of ‘social media’, Hippke’s and Lund’s claims have been highlighted as a refutation of the century-long dimming, and this has been extended to everything about KIC 8462852. For example, on the first day of the launch of the Kickstarter program, the Reddit talk had the statement that the person ‘thought this has all been refuted’.

Well, despite ‘social media’, it is actually Hippke and Lund that are definitely wrong. As I’ll show, any experienced worker can quickly find exactly what mistakes they made, so that their claimed large scatter of slopes arises simply from two distinct mistakes on their part. But I have no ordinary venue to put out any effective counters or proofs. For example, any further submission to ApJ or ApJLett would not have any new data to show, and it would appear only many months from now. Research on Tabby’s Star is moving fast, so Hippke’s and Lund’s claims need to be challenged soon. The best way that I can think of to get the challenge and proofs out is to place them into a detailed document plus an email (*this* email), and to send this out to people who have queried me for an analysis of Hippke’s and Lund’s manuscripts on Tabby’s Star. A link to the detailed document appears below.

I present three reasons to show that Hippke and Lund have incorrect claims:

Reason #1: Hippke & Lund Both Made Two Killer Mistakes

Mistake #1 is that they selected many check stars that have some random nearby star at just the right distance so as to produce overlapping star images on the Harvard plates with large plate scales. The DASCH photometry uses SExtractor, and the algorithm returns something like the combined magnitude when the two star images overlap. This overlap produces an erroneously-bright magnitude for some plates. This occurs for most of the plates after the 1953-1969 Menzel gap (the Damon plates), resulting in an apparent jump across the Menzel gap. When the whole light curve is fit to a straight line, it will also result in an apparently brightening light curve.

Some crowding stars will cause this effect to be mainly visible on the RB & RH series or the AM & AC series, which result in the opposite sign for the jumps and slopes. In the linked PDF file below, I give many detailed examples, tables, and illustrations. That is, jumps in brightness across the Menzel gap and non-zero slopes are produced as pure artifacts of choosing check stars with nearby crowding stars. Now, critically, Tabby’s Star does not have any crowding stars. So it is not correct to choose any crowded-check-stars. No experienced researcher would make such a choice. It turns out that a large fraction of both Hippke’s and Lund’s stars with high claimed slopes are badly crowded. That is, many of their stars have high slopes simply due to this bad mistake.

Mistake #2 is that they have used the KIC magnitudes for calibration, rather than the APASS magnitudes as strongly recommended by DASCH in many places. The KIC calibration is based on the ‘g’ magnitudes as used by the Kepler satellite, whereas the APASS magnitudes directly give ‘B’ magnitudes. The native system of the Harvard plates is ‘B’. So the use of the KIC-calibration will always be problematic for some purposes because there must always be color terms needing correction. It is only a historical relic that the DASCH database allows the use of the KIC calibration. Yet most of Hippke’s and Lund’s results were made with the KIC calibration.

This actually matters. The reason is that the KIC-calibrated light curve for some presumably-constant star often shows an apparent slope (and possibly a jump in brightness across the Menzel gap), whereas the APASS-calibrated light curve for the same star shows a perfectly flat light curve. I show several examples of this effect in the attached PDF file. With this, we see that the use of the KIC-calibration by Hippke & Lund is causing the jumps and slopes as pure artifacts. Their Mistake #2 would not be made by anyone experienced with the Harvard plates (or anyone who reads the DASCH website or papers).

The attached PDF file gives a detailed account of the commission of the errors. Between the two killer mistakes, all of Hippke’s and Lund’s claims are shown to be artifacts of their bad analysis.

Reason #2: Two Measures by Experienced Workers give ±0.044 and ±0.048

Measure #1 is by myself, as given in fine detail in my ApJLett paper. I derive the century-long slopes for 12 uncrowded check stars that have essentially identical magnitude, color, and position as Tabby’s Star. Whatever systematic and measurement errors happen for Tabby’s Star on the DASCH photometry, the identical effects must be present on these 12 stars. No one can do any better than this for a direct measure of the real total errors. With this, the average slope is very close to zero, while the RMS of the slopes is ±0.044 mag/cen. The largest deviation from a flat slope is one at -0.070 mag/cen. I should mention that I have a vast experience with the Harvard plates, with nearly continuous work since 1979, something like 50 papers in refereed journals, plus five papers on the theory of photographic photometry.

Measure #2 is by Josh Grindlay. He is a professor at Harvard; he has been a long time user of the Harvard plates (going back before 1979), and he is the founder and leader of the DASCH program. He had long been using DASCH light curves, so he knew perfectly well that DASCH produces flat light curves for constant stars. With the spectacle of Hippke’s paper, he started a formal measure of many Landolt stars with the DASCH data. (Landolt stars have long served the community as standard stars, and they are most likely closely constant in brightness.) For 31 Landolt stars, Grindlay finds that the average fitted-linear-slope is -0.015±0.048 mag/cen.

So we have the two most experienced workers in the world, and we are getting an RMS in the fitted-linear-slope of 0.044-0.048 mag/cen. For Tabby’s Star, this results in the century-long dimming being near 4.0-sigma in significance. I think that these two solid measures by the most experienced people in the world are to be strongly preferred to a claim coming from people who have yet to lay eyes on any archival photographic plate.

Reason #3: The Dimming of Tabby’s Star Has Been Confirmed

I recently received an email from Dr. Boyajian stating “I met with colleague Ben Montet, and he showed me his analysis of the Kepler Full Frame Images for KIC 846. It shows a pretty convincing dimming over the 4 yr time period (!!). He also shows that the dimming in our star is unique.” In a subsequent email, she stated “His method was pretty convincing, catching many things I wouldn’t have thought about. Doing this for a couple hundred other Kepler stars shows that the slope distribution is a Gaussian, and KIC 846 is an outlier.”

That is, in the 4.5 years of Kepler data, with a detailed analysis of the Full-Frame data, various known effects on the long-term light curves can be calibrated out, so a small amplitude overall fading can be recognized. Tabby’s Star is bright, and the Kepler data is legendary for its photometric accuracy and stability. If Tabby’s Star is fading at the rate of 0.164 mag/cen (which it might or might not still be doing), then it should have faded by 0.0073 mag over the Kepler lifetime on the main Cygnus Field. This should be discoverable by a careful analysis.

Apparently Montet has made such an analysis, and finds Tabby’s Star to be fading at some unspecified fade-rate. So we have an apparent confirmation of the fading of Tabby’s Star over 4.5 years, although certainly we must await a definitive paper coming from Montet.[A group at Pulkova Observatory has claimed to provide a weak confirmation of a fading of Tabby’s Star. This is based on just ten plates from 1922 to 2001.There is indeed a formally fading slope, but the real uncertainties are greatly larger than any claimed slope. This result is not a confirmation.]

For those interested in following this matter further, the document I discuss above, my “ANALYSIS OF HIPPKE et al. (2016) and LUND et al. (2016) is available.” Often the refutations of claims are not short, so I have presented the full details in this document. In sum: We have three strong reasons to know that Hippke’s and Lund’s claims are certainly wrong.

KIC 8462852 is a superficially ordinary main sequence F star for which Kepler detected an unusual series of brief dimming events. We obtain accurate relative photometry of KIC 8462852 from the Kepler full frame images, finding that the brightness of KIC 8462852 monotonically decreased over the four years it was observed by Kepler. Over the first ~1000 days, KIC 8462852 faded approximately linearly at a rate of 0.341 +/- 0.041 percent per year, for a total decline of 0.9%. KIC 8462852 then dimmed much more rapidly in the next ~200 days, with its flux dropping by more than 2%. For the final ~200 days of Kepler photometry the magnitude remained approximately constant, although the data are also consistent with the decline rate measured for the first 2.7 yr. Of a sample of 193 nearby comparison stars and 355 stars with similar stellar parameters, 0.6% change brightness at a rate as fast as 0.341 +/- 0.041 percent per year, and none exhibit either the rapid decline by >2% or the cumulative fading by 3% of KIC 8462852. We examine whether the rapid decline could be caused by a cloud of transiting circumstellar material, finding while such a cloud could evade detection in sub-mm observations, the transit ingress and duration cannot be explained by a simple cloud model. Moreover, this model cannot account for the observed longer-term dimming. No known or proposed stellar phenomena can fully explain all aspects of the observed light curve.

5.1. Comparison to DASCH Photometry

As mentioned in Section 1, Schaefer (2016) used 99 years of photometry from the DASCH project to analyze the behavior of KIC 8462852, finding a decrease in brightness of 14% from 1890 to 1989. Hippke et al. (2016) performed an independent analysis of the DASCH photometry, confirming that the photographic data yield a fainter magnitude for KIC 8462852 in the late 20th century compared to the end of the 19th century. However, Hippke et al. argue that the DASCH measurements from 1890 to 1952 are best described by a constant brightness and measurements from 1967 to 1989 are best described by a different (fainter) constant brightness, with systematic errors accounting for the offset.

We attempt to reproduce both results (for KIC 8462852 only, not the comparison stars). We find that either a linear decline in brightness with time or a constant brightness with a systematic offset between pre-1952 and post-1967 observations is a reasonable description of the DASCH measurements. The unfortunate gap of 15 years with very few observations in the middle of the century makes it difficult to distinguish between the two models. Which explanation to prefer then depends on one’s assessment of whether a star steadily fading for a century or a change in the photometric calibration of the Harvard plates around 1962 is more likely (or less unlikely). The fading that we detect from 2009 to 2013 with the Kepler FFI images does not necessarily represent a confirmation that KIC 8462852 also dimmed over the preceding 129 yr, but could make that interpretation of the DASCH data more plausible.

I'm a neophyte in studying deep space astrophysics, but I recently learned of something that reminds me – but maybe no one else – of Tabby's Star. This is Hubble's Variable Nebula (NGC 2261). As the nickname implies, this nebula varies in brightness (by 2 magnitudes, a factor of ~6), on a time scale of days, which is particularly interesting given that it is about 1 light year in radius, which would – one might think – cause variations in the central star's brightness to spread out across the nebula and average out. What is seen is, fascinatingly, superluminal motion, as the patterns of illuminated areas spread outward faster than the speed of light. It seems apparent that the variation in illumination from the central star is direction-specific, so that patterns of light and dark move through/across the nebula as those radial variations take place. A suggestive possible analogue is someone holding a flashlight surrounded by sheets of gauze, making patterns move at a distance across the gauze as they turn the flashlight.

Undoubtedly, Tabby's Star and NGC 2261 have some things in common, but whether those things are superficial or meaningfully related is another matter. I haven't seen anyone suggest a relationship between them, and NGC 2261 is itself mysterious in nature, although some plausible guesses as to its nature exist. What's interesting about using NGC 2261 as a starting point is that we can resolve the nebula pretty well, whereas Tabby's Star has only been amenable to photometry.

Undoubtedly, Tabby's Star and NGC 2261 have some things in common, but whether those things are superficial or meaningfully related is another matter. I haven't seen anyone suggest a relationship between them, and NGC 2261 is itself mysterious in nature, although some plausible guesses as to its nature exist. What's interesting about using NGC 2261 as a starting point is that we can resolve the nebula pretty well, whereas Tabby's Star has only been amenable to photometry.

Another superficial, but related idea- close encounters between centaurs (minor planets) and gas giants result in centaurs with rings.

http://aasnova.org/2016/09/02/rings-from-close-encounters/"In these cases the icy mantle and even some of the centaur’s core can be ripped away and scattered, becoming gravitationally bound to the largest remaining clump of the core. The particles travel in highly eccentric orbits, gradually damping as they collide with each other and forming a disk around the remaining core. "

I took a picture of it a few months ago, and I could take another picture now with the same camera parameters and compare the difference. That'd make for a nice blink GIF. It will have to be a pretty big dimming event to be noticeable, though, and to exceed differences in seeing conditions, etc.

Official monitoring program page, although it's not updated very often--latest was May 19 with the announcement and a plot of photometry showing 2.5% dip so far. There's a bit more chatter on Twitter with some APO (Apache Point Obs?) spectra; I'm not sure how to interpret it. Optical spectra experts care to comment?

By the way, at the same time and almost the same part of the sky, there is, coincidentally, a supernova currently visible in galaxy NGC 6946, and it's very convenient to see both of these episodes in one session.

The spectral analysis of this dimming event may likely resolve the nature of the mystery, if any thermal/compositional signatures are seen – or aren't. I'd place my wager on material in the interstellar medium coincidentally between us and the star, but the endogenous explanations (or lack of explanation… this would seem to be a never-before seen phenomenon) make a good case, too.

The spectral analysis of this dimming event may likely resolve the nature of the mystery, if any thermal/compositional signatures are seen – or aren't.

Looks like "aren't" at least initially...

QUOTE (http://www.astronomerstelegram.org/?read=10406)

We report medium resolution spectroscopy (R=2500) taken with the FRODOSpec fibre fed integral field spectrograph of the 2.0 meter Liverpool Telescope, La Palma obtained on 20th May 2017 starting at 01:20UT. Three 600 second exposures were obtained, giving a total integration time of 1800 seconds. The wavelength range was 5800 - 9400 Angstroms. ...In an initial analysis we find no difference between the two spectra...

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